1. Field of the Invention
This present invention concerns a control device for a multiphase inverter connected to a power supply source through a DC circuit and controlled in pulse width modulation through a control circuit, including a bridge consisting of several pairs of static switches placed in parallel with free wheel diodes, the mid-point of each pair being connected to a load; switching is done by comparison of an input signal and a triangular carrier wave in the control circuit.
2. Description on the Prior Art
The purpose of an inverter is to generate a power supply, for example a three-phase supply with variable voltage and frequency starting from a dc source. The inverter switches the DC source and outputs voltages that contain fundamental components with phase shifts so as to form a balanced three phase power supply that can be used to power a load, for example a motor.
The switching system usually used, that will be defined as a standard switching, uses a fixed switching frequency. It consists of choosing a switching period to define switching times for pairs of switches on the inverter bridge, by comparing an input signal and a carrier wave, the frequency of which is equal to the switching frequency.
However this switching generates noise, which results in the appearance of frequency bands at multiples of the switching frequency.
Thus a high switching frequency considered to be beyond the range of audible frequencies, will create high switching losses.
At low switching frequency within the range of audible frequencies, this noise may simply be annoying to the human ear.
One known method of reducing noise is to spread the frequency bands that appear at multiples of the switching frequency, within the spectral range.
This is usually done by using a system with a random switching frequency. The frequency is determined at random within a given frequency range. If the shape of the carrier wave is triangular, this system with a random switching frequency can create triangles with a variable period. This system is suitable for medium switching frequencies.
On the other hand at low frequencies, it is difficult to apply this system with random switching frequency. The frequency band is narrow and therefore the number of frequencies to be drawn at random is small.